1. Field of the Invention
The present invention relates to an apparatus for use in a circular charged particle accelerator or a particle accumulation ring and capable of detecting the position of an accelerated charged particle precisely and quickly with a high degree of sensitivity.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a position detector and a signal processing circuit which are used in a known charged particle position detection apparatus of the type which is disclosed, for example, in an article presented by T. Ieiri et al., in IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4 (Aug., 1983), pp 2356-2358. Referring to this Figure, a position detector 1 is used in a particle accelerator such as a circular charged particle accelerator, a particle accumulator ring and so forth, and is capable of detecting the position of a charged particle passing through a vacuum duct. The detector 1 includes a plurality of electrodes secured to the vacuum duct and capable of picking up the position of the passing charged particle as an induced charge. In the illustrated case, four electrodes 2 to 5 are used. Transmission lines 6 to 9 are connected to the respective electrodes 2 to 5 so as to transmit signals from these electrodes to circuits for processing these signals. Mixers 10 to 13 with filters are electrically connected to these transmission lines 6 to 9 and also to an oscillator 14. These mixers 10 to 13 are capable of picking up the second higher harmonic components of passage frequencies produced when a charged particle passes by the electrodes 2 to 5.
FIGS. 2 and 3 are sectional views of a known position detector which is used as the position detector 1 shown in FIG. 1 and which is disclosed, for example, in Proceeding of The 5th Symposium on Accelerator Science and Technology, 154-156, 1984. In these Figures, a pipe 15 forms a vacuum duct or a vacuum chamber, while 16 denotes an orbit of a charged particle.
FIG. 4 is a sectional view of another position detector of the type which is proposed by Tomotaro Katsura and Shinkichi Shibata in Beam Position Monitor for the Photon Factory Storage Ring. The position detector 1A has, in addition to electrode plates 2 to 5, a BNC connector 17 for externally picking up detection signal indicative of the position of the charged particle, and a supporting guide 18 supporting the BNC connector 17. Numeral 19 denotes a charged particle, while 16A denotes the central axis of the charged particle 19. An electrode 2 is supported by a supporting guide 18 together with the BNC connector 17. When a charge is induced by an electric field formed around the charged particle 19, the electrode 2 picks up this charge as the detection signal indicative of the position of the charged particle, and delivers this detection signal to the BNC connector 17. Similarly, other electrodes 3 to 5 are supported by the respective supporting guides (not shown) and deliver detection signals to the BNC connector 17. The BNC connector 17 is grounded commonly with the vacuum chamber, i.e., the pipe 15.
The operation principle of the known charged particle position detectors shown in FIGS. 1 to 3 is as follows.
When a charged particle passes a certain position in the orbit 16 within the vacuum duct, charges are induced in the electrodes 2 to 5 as functions of the distances between the above-mentioned certain position and the respective electrodes, whereby voltages are generated in these electrodes. These voltages are delivered to the mixers 10 to 13 with filters through the transmission lines 6 to 9 so as to be processed. In this case, the position x, y of the axis of the charged particle on a fixed coordinate is related to the voltages V.sub.1 to V.sub.4 induced in the electrodes as follows. EQU x.varies.(V.sub.1 +V.sub.2 -V.sub.3 -V.sub.4) EQU y.varies.(V.sub.1 -V.sub.2 +V.sub.3 -V.sub.4)
It is therefore possible to know the position x, y of the charged particle by means of measuring of the induced voltages V.sub.1 and V.sub.4. The induced voltages V.sub.1 to V.sub.4 are delivered through the transmission lines 6 to 9 to the mixers 10 to 13 in which signal components having frequencies which is twice as high as the passage frequency are picked up and the coordinates x and y are determined in accordance with the formulae shown above using these signal components.
By comparing the quantities of the charges induced in the electrodes 2 to 5, it is possible to detect the position of passage of the charged particle.
In the known position detector 1A shown in FIG. 4, charges are induced in the electrodes 2 to 5 by an electric field formed around the charged particle 19 moving through the vacuum duct. Voltages formed as detection signals by the charges induced in the respective electrodes 2 to 5 are represented by V.sub.1, V.sub.2, V.sub.3 and V.sub.4. The movement of the charged particle 19 moving through the vacuum duct is simulated as shown in FIG. 4 and the relationships between the x, y coordinates (x), (y) of the charged particles and the following values determined by the voltages V.sub.1, V.sub.2, V.sub.3 and V.sub.4 are obtained in advance to form calibration curves as shown in FIG. 5. EQU [(V.sub.4 +V.sub.1)-(V.sub.2 +V.sub.3)]/(V.sub.1 +V.sub.2 +V.sub.3 +V.sub.4) EQU [(V.sub.1 +V.sub.2)-(V.sub.4 +V.sub.3)].(V.sub.1 +V.sub.2 +V.sub.3 +V.sub.4)
It is therefore possible to find the position of the charged particle 19 from the measured values of the voltages V.sub.1, V.sub.2, V.sub.3 and V.sub.4 by consultation with the calibration curve shown in FIG. 5.
In the known charged particle position detection apparatus shown in FIG. 1, the detection signals from the detector are transmitted to processing circuits through the transmission lines. It is therefore necessary to use a multiplicity of transmission lines, e.g., four transmission lines as illustrated. In addition, components such as mixers with filters have to be used in the signal processing circuits in numbers corresponding to the number of the signal transmission lines. It is necessary to eliminate any fluctuation or variation between the components of the systems connected to different signal transmission lines, with the result that much labor is required.
Problems are also encountered in that the quantity of the charges detectable is limited due to a quick change in the polarity of the induced charges as a result of passage of the charged particle, and in that the quantity of charges induced varies depending on the momentum of the charged particle.
The known position detector of the type shown in FIG. 4 also suffers from a problem in that, since the BNC connector is grounded together with the vacuum duct, and since the vacuum duct function as a kind of antenna so as to pickup external noise, the detection signals derived from the BNC connector tends to be disturbed by the noise transmitted from the vacuum duct. When the amount of charge of the charged particle is small, the level of the detection signals from the position detector also is reduced to a level lower than the level of the noise so as to become insensible. In such a case, it is impossible to detect the position of the charged particle.